Preliminary assessment of mud crab stocks in Bua Province

By Yashika Nand

Mud crabs are a popular dish in Fiji that over the years has become a luxury food item. Single large mud crabs will cost you FJ$50-70 per crab or FJ$100-150 for strings of crabs at the Suva market, with the larger ones selling out very quickly. Mud crab dishes can cost you FJ$40-50 per dish.

A woman mud crab fisher. Credit:WCS

A woman mud crab fisher. Credit:WCS

There is a general perception that stocks around Viti Levu have been depleted while those in Vanua Levu are still healthy. However, we don’t have any data to verify this perception. We also do not know if we are harvesting too many female crabs with eggs or too many undersized crabs. How many crabs do we need in a mangrove forest to enable the local population to continue and thrive? These were some of the questions the Wildlife Conservation Society (WCS) are fishing for answers for

In July 2016, WCS partnered with the female representative for the Fiji Locally Managed Marine Area (FLMMA) network, Department of Fisheries and women mud crab fishers to trial a method to determine the density and biomass (or weight) of mud crabs in different mangrove forests in Bua Province. Our early discussions with these fishers taught us that mud crabs inhabit three major habitat types – mangrove systems, intertidal mudflats, and coral reef systems.

After discussion with a number of international experts, we decided to trial a survey technique that combines a “depletion method” with “transect surveys”. Basically, very experienced mud crab fishers were asked to collect all the crabs they can find within a fixed and marked area (square) 400m2 in size. Each crab that is caught is tied and at the end of the survey measured to get the carapace (shell) length and total length, weighed and then released back into the mangroves or in some cases, kept for sale or consumption.

WCS Conservation Scientist, Margaret Fox. Credit:WCS Fiji.

WCS Conservation Scientist, Margaret Fox. Credit:WCS Fiji.

We conducted mud crab surveys in the districts of Dama, Navakasiga and Lekutu. We took a selected our sites based on the following criteria: (i) fishing pressure; (ii) density of mangrove systems; and (iii) types of mangrove forest. What we did not anticipate was how intensive it is to conduct surveys in Rhizophora forests, which are known for their complex array of aerial roots. It took us almost 45 minutes to mark the boundaries of one quadrat, especially for us town dwellers who are not used to walking through mangrove forests.

Over 7 days, we surveyed 30, 800m2 of coastal mangrove forests and developed a deep appreciation for the hard work that goes into catching mud crabs, and the skills required to find the crabs. The women we worked with walk barefoot through mangrove forests, looking for crab holes, which scattered in no real pattern.

We learnt that mud crab fishers spend hours in a mangrove forest looking for crabs that we enjoy eating, most only catching 2-10 on the low tide. Our preliminary analyses suggest that mangrove forests are a habitat for majority adult and some juvenile and sub-adults. The densities we recorded in Bua Province were slightly lower than that those that have been documented in New Caledonia and Australia. However, we were tide constrained and the challenging habitats we had to survey and scattered distribution of mud crabs meant we might have slightly underestimated the population status. However, the surveys conducted are a good baseline for understanding the status of the stocks and the fishery, especially if we link it to catch rates and volumes by local fishers. And most importantly, the surveys allow us to engage directly with mud crab fishers to help them assess the stocks in their mangrove forests.

Understanding what state the stocks are in, is a good first step to understanding what management measures need to be in place to ensure the fishery is sustainable for the long-term, and Fijians can keep eating the mud crabs they love!

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